Ringing equipment for pulse modulation systems



April 3, 1951 G. P. wENNl-:MER

RINGING EQUIPMENT FOR PULSE MODULATION SYSTEMS 2 Sheets-Sheet 1 Filed Oct. 2. 1947 ATTORNEY /NVE/VTR G. E WEA/NEWER @A ANI April 3, 1951 G. P. WENNEMER RINGNG EQUIPMENT FOR PULSE MODULATION SYSTEMS l2 sheets-shed 2 Filed Oct. 2, 1947 INHWL (Ill '/Nl/ENTO/Q c. R WEA/MEME@ ATTORNEY Patented Apr. 3, 1951 RINGING EQUIPMENT FOR PULSE MODULATION SYSTEMS Gerard I. Wennemer,

to Bell Telephone La FairmountyN. J., assignor boratories, Incorporated,

New York, N. Y., a corporation of New York Applicationctober 2, 1947, Serial No. 777,455

are transmitted. At the receiving station means are provided for recovering the message signals from the time-modulated pulses of each series. fn such systems it is necessary to provide some means for sending supervisory signals suchkas ringing signals between stations whereby an operator at the receiving station maybe notified when a message is to be transmitted and given information as to which channel or channels will be used for the transmission.

Heretofore, ringing signals have been transmitted over pulse position modulation radio systems by eliminating at the transmitter, the channel pulses corresponding to the signal channels for which ringing is desired. Inl such systems means have been provided at the receiver for actuating the ringer whenever the pulses of a channel are absent in the transmitted signal. This method of ringing is not completely satisfactory when employed in a multi-channel system wherein the radio link between the transmitting and receiving station includes certain types of velocity variation tubes as the transmitting oscillator. It is a characteristic of such velocity variation tubes that their operating frequency may be varied materially by changes in the duty cycle of the modulating signal applied thereto. Consequently, the removal of a channel pulse in the multiplex signal applied yto the radio transmitter causes a change in the operating frequency of the oscillator and if ringing is desired simultaneously in several channels, the change in the duty cycle caused by the elimination of 'the channel pulses for a plurality of channels is sufficiently great to produce large changes in the operating frequency of the transmitting oscillator. If, for example, ringing is effected simultaneously over several of the channels of an 8channel system employing a radio frequency of 5,000 megacycles, frequency changes of the order of 25 megacycles may be produced. As a result, the radio receiver must have a wide range automatic frequency control if the transmitted signals are to be received. Such a receiver is suslill 5 Claims. (Cl. 179-15) ceptible to extraneous noise and unwanted signals.

it is the object of the present invention to provide ringing equipment for time division multiplex, pulse modulation systems, such that no changes in the duty cycle of the radio transmitter will be produced by the ringing operation.

In accordance with the invention ringing is accomplished by shifting the positions ofthe pulses of the channel in which ringing is desired to such an extent that the normal range of modulation produced by message signals is exceeded. At the receiver, means are provided for operating a ringer whenever the channel pulses of a particular channel fall beyond the range of positions reserved for message signal modulation. The above and other features of the invention will be described in detail in the following specification taken in connection with the drawings in which:

Figs. 1A and 1B are block diagrams of the transmitting and receiving portions respectively of a time division multiplex, pulse position modulation system with ringing equipment according to the invention;

Fig. 2 is a schematic diagram of a portion of the transmitting equipment of Fig. 1A showing in detail the ringing equipment for a single channel; and

Fig. 3 is a schematic diagram showing the ringing equipment required at the` receiving station for a single channel of the receiving equipment o f Fig. 1B.

A brief description of a time division multiplex pulse position modulation system will be helpful in showing the interrelation between certain of the message circuits and the ringing equipment. A typical system is shown in block form in Figs. 1A and 1B. In this system interlaced series of pulses are modulated in their times of occurrence or time phases in accordance with the message signals to be transmitted over each channel. In Fig. 1A, eight input message channels identied as CI-l-l through CH-B are shown. The channels are similar and channel No. 4 will be taken as representative for purposes of description. In this channel either the incoming message signals or ringing signals, which may for example be a .20-cycle alternating current, are applied to an input network l0 which serves both to separate the message signals from the low frequency ringing signals and to limit the range of frequencies in the message signal to the range acceptable by the modulating equipment. The message signals are applied through an ampliiier lvl to a pulse modulator I2, while the ringing signals are applied over a separate path directly to the pulse modulator.

The eight pulse modulators receive short impulses at xed times during the multiplex period and in response to these impulses produce lengthmodulated pulsesv which are timed normally to* end at the center or mid-point of the channel period assigned to the particular pulse modulator. The time-modulated pulses so produced are varied in length by the message signal currents'. applied to the modulators from the input channels. The endings of the length-modulated pulses from the pulse modulators are employed to energize pulse generating devices to produce pulses, the time positions of which depend upon the length of the pulses received from the pulsemodulators. These time-modulated pulses are applied to a radio transmitter and employed to control the radiation of corresponding pulses of ultra-high frequency energy.

The pulse modulators are energized by pulses derived from a master oscillator I3, the output of which is differentiated to obtain a series of positive and negative pulses. Since it is desired to energize the eight pulse modulators serially during one multiplex period, the output of the master oscillator I3' is applied to differentiating circuits I4', i4 and I 'I to obtain positive and negative pulses which are 180 degrees out of phase. Thus, by choosing the proper differentiated pulses and reversing' their polarity as required by means of exciters I5 and' I5', the operation of the several modulators may be controlled by the oscillator I3. Since a slight interval or guard time must be allowed between the channell signals to permit separation of the channel pulses at the receiver, the maximum range of position modulation is made slightly less than the channel period allotted to each channel.

The length-modulated output pulses from the pulse modulators of odd-numbered channels are applied to a pulse generator I8" while those' for the even-numbered channels are applied to a similar pulse generator I8 and the position modulated pulses produced by these pulse generators are applied to a radio transmitter I9. Oscillator I3 is also employed to operate a marker generator to produce a marker pulse timed to precede the position-modulated pulses from pulse formers I 3 andv t8.

Ringing signals which are separated from message signals by the input networks in each channel, are applied directly to the pulse modulators wherein they are employed to vary the length of the length-modulated pulses produced by the associated pulse modulators to an extent beyond theA variation produced by the maximum amplitude message signal. The` channel pulses produced by the pulse generators under such conditions-fall within the guard time between adjacent channels. The` input signal to the transmitter thus includes av pulse for each channel irrespective of whether a ringing signalr is` present in any channelv and the dutycycle of the transmitting oscillator remains the same.

' Thereceiving equipment of the system is shown in Fig. 1B. I-Iere the radio frequency pulses are rectiiied in radio receiver 56 and applied through ampler 5I to two different paths. In one path thev signals are applied through amplier 56 to input circuits corresponding to the eight message'Y channels. In each of these channels and, for' example, in channel l` the radio frequency signalsfor all channel'sare-applied through highv resistances such as resistor 51 to the input of a pulse converter, such as pulse converter 58. In the other branch the received and rectied radio signals are applied to a marker selector 52 Which separates the marker pulses from the remainder of the signal and applies the marker pulses to a square wave generator. 54- This square wave generator is employed to control eight gate generators, one for each channel, which are arranged to produce gate pulses coincident and coextensive with the portion of each of the channel periods corresponding to the maximum range of position modulation. Considering the fourth channel, the output of the square wave generator 513 is suppliedto: a gate generator 55, the output of which is applied to pulse converter 58.

Pulse converter 58 is responsive only when the time position-modulated channel pulses and the gate pulses are present at the same time and is arranged to produce a pulse the length of which is dependent upon` thel time position of the channel pulse applied through resistor 5l. These length-modulated pulses are passed through a low-pass' lter 59 to recover the message signal which enters amplifier 60, the output of which is applied toswitching equipment shown diagrammatically at 6I.

The output signals-from` the pulse converter are also applied through a rectier 62 to ringing equipment 531 The rectified currents applied to the ringing equipment are-employed to maintain the ringing circuit disabledv soA long as both gate and channel pulses are applied simultaneously to theV pulse converter. Upon failure of a channel pulse to-reach the pulse convertersimultaneously witha gate pulse' the rectified current falls tov such a` value that the' ringing circuit is actuated. Since the application of a ringing signal to any channel at the transmitter causes the channel pulses for' that channel to4 occur in' the guard time between adjacent channels, and thus at times outside thel range to which the gate pulses correspond, a drop in the re'ctied current will occur and the ringing circuit for the corresponding' channel at the receiver will operate.

Also'showir in Fig. 1B' is a marker alarm circuit actuated by the output of marker selector 52 and comprising an amplifier 64Y and` alarm G5. Upon failure of the markery pulses the marker alarrn` which is normally disabled by thev output of amplifier 64 is operated andV in i's applied over leadl t6` to each of the ringing equipments, such as 63 of channel 4 to disable the ringing equipment thus to prevent ringing on all channels in1 the event of failure of the marker pulses.

A more complete'description of this typical systerny may be' found at page 82- of Electronic Industries for December 1945, vol. 4, No. 12 or in the applica-tion of Ji O; Edson,v Serial No. 559,354, iiled= October 19, 1944', entitled Multiplex Teleph'ony.

Details of the4 ringing equipment for the transmitting andr receiving stations of the system of Figs. l-A andv 1B.rare shown in Figs. 2 and' 3, respect-i-Vely, each of which represents appropriate portions of` a single message channel. Referring to-Fig. 2, the signal input for the channel which may comprise either amessage signal or a low frequency' ringing signal is applied to an input transformer IlUIlinwhichsignal and ringing currents-r are separated. The' signal current output path includes the secondary Winding of transformer |00, a level: adjusting' pad |02 and a lowp'ass iilterl IIMy which limits the frequency range addition a signalv output from the amplier 1s limited by series resistor |42 in thegrid circuit in one direction" and the cathode resistor in the other. The maximum output signal is thus limited to corre'- spond tothe maximum range'acceptable by the :fr-iodulatorto which itis applied.

' `The pulse" position modulator comprises triode tubes ||4 and H6' connected in a relaxation circuit of a type sometimes' known "as one shot or single-trip multivibrator. The plate of tube H4 isV connected through capacitor ||B` to the control grid of tube IE6 to furnish the intertube connection in one direction While the cathodes oi' the two tubes are connected together and through a common cathode resistor to ground to furnish the intertube connection in the other direction. The values of the coupling circuit components including capacitor H8, variable resistor |22 connected in the plate circuit of tube H4, bias resistors |24 and |26 connected in the grid circuit of tube H6 and the common cathode resistor |20 a're so chosen that the circuit does not oscillate and that tube ||6 normally conducts while tube I4 is cut oi. Upon receipt of a positive pulse applied from the master oscillator through an eX- citer tube |23, current flow is initiated in normally cut-off tube ||f| and a negative pulse, corresponding to the drop produced across resistor |22, is applied through capacitor ||8 to the control grid of normally conducting tube H6. Be-

cause of the coupling through common cathode resistor |20, reduction in current through tube H6 proceeds rapidly and the entire current is transferred to tube H4. Current flow continues through tube ||4 until capacitor ||8 is charged through resistors |26 and |24 to a sufficiently high potential to permit current flow to be reestablished through tube H6, at which time the circuit revertsvto its normal condition. The valuesl of capacitor ||6 and resistors |24 and |26 are so chosen that the circuit returns to itsv normal condition at a time which corresponds to the mid-point on the channel period for the channel with which the modulator is associated. Modulation of the length of the output pulse obtained from the single-shot multivibrator is effected through the application of the output from message signal amplier |06 to the charging circuit for capacitor H0. Thus a portion of the output signal oi? tube |05 appearing across series resistors '|26 and |30 is applied to the control grid of tube ||6 through resistor |24. rlhe charging time for capacitor ||8 is accordingly advanced or retarded with a correspondingerfect upon the length of the output pulse of the one shot multivibrator. The output pulses of the one shot multivibrator are applied 'through an output tube |32 to apulse generator in which the length-modulated pulses are converted to position-modulated pulses for application to the radio transmitter, the time positions of the pulses from the pulsegenerator being dependent upon the lengths of the pulses applied thereto.

Ringing signals, when applied to'input transformer |00, appear across capacitor ||2 vwhich oiiers a relatively high impedance at low frequencies and are applied to a rectifier |34, the direct current output of Which is employed to actuate a relay |36. Armature |30 of relay |36 normally completes a connection from the output of lter |04 through contact |40 and resistor |42 tothe control grid of message signal ampliiier |06. Upon actuation of the relay in response to ringing currents the connection of the grid to contact |40 is transferred to contact |44. When this circuit connection is established, the output oi lter |04 is removed from the grid circuit of the message signal amplifier and a bias voltage obtained from divider |46 is applied thereto At the same time, armature |48 corn- -pletes a circuit to connect a resistor |52 in parallel with resistor |30 in the anode circuit of the message signal amplifier. The net result of these changes in the connections to message signal -ampliiier tube |06 is to change the output voltageapplied through` resistor |24 to capacitor ||3 of the single-shot multivibrator and to lengthen the length-modulated pulse produced by the single-shot multivibrator beyond the range of length variation that may be effected by the maximum message signal transmissible through amplifier |06. As a result, the postionemodulated pulse produced by the pulse generator represents a shift in position beyond the range assigned to the message signals, and the resulting output pulses occur in the interchannel guard time.

At the receiving station, the position-modulated pulses are applied to control a single-trip multivibrator similar to that employed in the transmitter and comprising triodes |54 and |55. As in the case of the single-trip multivibrator at the transmitter, the anode of triode |54 is connected through capacitor |58 to the control grid of triode `|56 While the cathodes of the two tubes are connected together and through a common cathode resistor |60v to ground. The values of the circuit components including resistor |60, capacitor |53 and grid resistor |62 of triode |56 are so adjusted that current normally flows through tube |56. In addition, these components are so'chosen that the position-modulated pulses alone applied to the control grid of normally non-conductive tube |54 are insufficient to cause transfer of current from tube |56 to |54. Howevena gate pulse derived from the marker pulse and coextensive with the portion of the channel period corresponding to the maximum range of position modulation is also applied to the control grid of tube |54. In the presence of both the channel and gate pulses, the control grid of tube |54 becomes suiiiciently positive to cause transfer of current from tube |56 to tube |54. The time constant of the charging circuit for capacitor |58 is made large With respect to the time of the channel period with the result that current flow does not revert to normally conductive tube |56 for the remainder of the channel period. Therefore, the output of the single-shot multivibrator consists of a pulse, the length of which is dependent upon the time or occurrence of the time position-modulated pulses applied to/the control grid of tube |54. These length-modulated pulses are applied through a capacitor |64 to a low-pass filter We the output of which is applied to an amplier |08. The output of the low-pass lter andthus that of amplifier |62 is a reproduction of the signaling currents applied at the transmitter and may be applied through a transformer |10 to an output circuit which is normally completed through contacts |'l2 and |14 of a relay |16.

The outputV pulses from the single-shot multivibrator are also applied through a capacitor' |15 to a ringing circuit. In this circuit the pulses are rectified by the shunt rectifier IBB to obtain negative pulses which are applied to charge a capacitor |82 in such polarity as to apply a negative bias to the control grid of triode tube |84. The anode circuit of tricde |84 is completed through the winding of relay |16 and through the marker alarm` equipment to a source of positive potential. The circuit constants of triode Y|84 are so adjusted that the negative bias produced across capacitor |82 by the output pulses from single-trip multivibrator |148 is of suicient magnitude to prevent the now of current therethrough, the time constant of the RC circuit being such as to hold theA bias between successive output pulses. If, however, the output pulses from the single-*shot multivibrator are interrupted the negative bias disappears and triode |84 draws anode current operating relay |16 to connect a ringing generator E86 to the output circuit. Since the single-shot multi vibrator is responsive only when channel pulses and gate pulses are applied simultaneously to the control grid of tried-e i54v it will be recognized that whenever the time position of the channel pulse is shifted beyond the maximum signal range in response to the application of a ringing current at the transmitter station, the ringing circuit at the receiver will be actuated to transmit ringing current from generator |85 to the output circuit of the corresponding channel.

What is claimed is:

1. In a pulse modulation system, means at the transmitter for producing a series of pulses, means for modulating the relative time positions ofthe pulses of said series in accordance with a signal to be sent to a receiving station, means at the receiver for producing a local pulse of length equal to the maximum range of position variation produced by the modulation of pulses of said series at the transmitter, means at the receiver for producing a supervisory signal unless the time-modulated pulses from the transmitter coincide in time with said local pulses, auxiliary means at the transmitter for shifting time positions of the signal pulses to positions beyond the maximum displacement produced by signal modulation, and means for operating said auxiliary means whenever a supervisory signal is desired. Y

2. In a multichannel time division pulse modulation system, means at the transmitter producing. a plurality of interlaced series of pulses, meansfor each channel for modulating the rela-I tive time positions of the pulses ofr one of said series in accordance with a signal to be sent over said channel, means at the receiver for producing synchronized local pulses for each channel, said local pulses being of length equal to the maximum range of position variation produced by the modulation of the signal pulses at the transmitter, means at the receiver for each channel for producing a supervisory signal un less the signal pulses in thev channel coincide in time with the local pulses for that channel, and means at the transmitter for shifting the signal pulses of a channel in which a supervisory signalis desired to positions beyond the maximum displacement produced by signal modulation.

3. Ina time division multiplex system in which 7 a time. period is periodically assigned to each channel and signals are transmitted as time modulations of 'slior't pulses occurring Within said time periods, a plurality 'of input message channels, a relaxation circuit for each channel having a normal and a transient conditionA of stability, means for switching each relaxation circuit to the' transient condition of stability at the beginning of the time period assigned to the channel with which it is associated, means responsive to the' message signal in that channel for switching said relaxation circuit back to its normal condition of stability, means for applying a ringing signal to each input channel and means responsive to said ringing signal for retarding the return of said relaxation circuit to its normal condition of stability for a time greater than the maximum delay produced by sa-id message signal.

4; In a time division multiplex system inwhich a time period is periodically assigned to each channel and signals are transmitted as time modulations of short pulses occurring within said time periods, a plurality of input message channels, a relaxationcircuit for each channel having a normal and a( transient condition of stability, means for switching each relaxation circuit to its transient condition of stability at the beginning of the time period assigned to the channel with' which it is associated, means responsive to the message signal in that channel for switching said relaxation circuit back to its normal condition of stability, means for applying a ringing signal to each input channel, means responsive to said ringing signal for retarding the return of said relaxation circuit to its normal condition of stability for a time greater than the maximum delay produced by said message signal, and means at the receiving station for each channel for producing agate pulse coincident and coextensive with the range of time variation produced inthe relaxation circuits of the transmitter by said message signals', a normally operating alarm circuit and means responsive to the sux'ri of s'aid gate pulse a'r'id the message pulse forl each channel for' cutting off said alarm circuit.

l 5. In a pulse' modulation system means for producing a sries of pulses", means for modulating the relative tir'iie positions of said pulses in ace' corda'nce with a` message signal to be transmitted, the position modulation of each' pulse being limited to' an assigned modulation time separated from adjacent modulation times by guard times, an auxiliary circuit at the transmitter 'for' shifting the time positions of the signal pulses intosaidl guard times", meansV for actuating said circuits when' a' supervisory signal is desired,

and means' at the receiver for producing. a

supervisory signal output whenever signal pulses occur atv other than the' corresponding modulation times; Y A GERARD P. WENNEMER.

REFERENCES CITED i The following references are of rec'ord in the fil-e of this patent:

UNITED STATES PATENTS Number a'me Date' 2,262,838 Deloraine Nov. 18, 1941'v 2,416,330' Labin Feb. 25, 1947 2,432,204 Miller' f Dec. 9, 1947 2,438,902 Delorainev v v e Apr. 6, 1948- 

